To elaborate on the above now that I'm at a computer...
Here's a list of ways debris/contaminants get in your filament:
- From the barrel/auger wearing against each other
- From the pellets themselves
- From dust in the air
- From dirt on the cup/bag used to put the pellets in the hopper
- From corrosion on the barrel/auger
Here is a list of causes that are taken care of by perfect centering and radial constraint of the screw in the auger:
- From the barrel/auger wearing against each other (note: the auger experiences several hundred pounds of axial load, and the barrel/auger clearance is half a millimeter. It is very likely that the auger will deflect 0.5mm as a result of this loading, nullifying any attempt at stabilization.
However, centering/constraining the auger in the barrel would require at minimum a bearing or a bushing at both ends, meaning one end is in the path of the polymer. In addition to making things much more complicated to machine, and obstructing the flow path for a very viscous polymer, you have one of two problems:
1.) Bushings wear. These wear particles would be dumped right back into the polymer, negating the whole point.
2.) Bearings are typically not designed to withstand 200C operating temperatures, most normal lubricants can't handle this (so add $$$)
On the other hand, here's a list of causes that are solved with a melt filter:
- From the barrel/auger wearing against each other
- From the pellets themselves
- From dust in the air
- From dirt on the cup/bag used to put the pellets in the hopper
- From corrosion on the barrel/auger
A melt filter is also much easier to machine, and is optional (does not increase cost if the user does not need it, which is the case if they're using nozzles bigger than 0.6mm).
You tell me which approach is better.